Neosecurinane alkaloids

Scheme 9 General skeleton of neosecurinane alkaloids (numbering).

More recently, two additional neosecurinane alkaloids, virosine A (22) and B (23), were isolated from Flueggea virosa by Ye. The case of virosine A (22) is intriguing. Indeed, the proposed stmcture for virosine A (22) referred to securinol B but after careful analysis it is evident that the reported data for these compounds are different ( H NMR shifts and [a]o values). Moreover, these compounds were isolated from plants known for producing enantiomers, thus the probability to extract the same enantiomer from these sources is almost impossible. Besides, the proposed structure for virosine A (22) was supported by detailed 2D NMR and CD analysis, whereas the revised structure stipulated for securinol B was extrapolated from the revised structure of securinol A (21), which was secured by X-ray crystallography. In conclusion, from the actual data, one can propose that the structure of virosine A corresponds to that depicted above, in other terms virosine A (22) is the C8-epimer of securinol A (21). Concerning securinol B, additional studies are needed to establish its structure. 

Besides the typical securinane structure, a new skeleton was identified in which the ring A is attached to position Cl 5 on the BC unit. The resulting framework displays an ethylene bridge and thus was named neosecurinane. This section wiU present the different alkaloids possessing this particular backbone. The carbon numbering on the neosecurinane skeleton follows that depicted in Scheme 9. 

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